Various apparatus for this purpose are known. For example, it has already been proposed to operate with individual grinders or abrasive blocks, but they have the drawback of tending to follow the undulations of the rail against which they are applied. In order to overcome this disadvantage, shoes of a certain length have been used which support several individual abrasive blocks or grinders in a rigid linear array extending in the direction of the track. By virtue of their length, these shoes tend to diminish the disadvantage mentioned above although, depending on the length of the shoe relative to the length of the undulations, the shoe may follow the latter, if, as is usual, it has an articulated suspension from the carrying vehicle.
It has also been proposed to use shoes which support grinders at distances which are variable longitudinally of the rail direction in order to avoid the resonance phenomena which can occur with certain spacing of the grinders for particular lengths of rail undulations.
In order simultaneously to eliminate the shortest as well as the longest of these undulations, it is necessary to provide very long shoes having a great number of points at which they contact the rail. These points may be provided by abrasive blocks of grindstones, and also by rail-following wheels, shoes or other non-abrasive sensors.
However, by increasing the number of rail contacting points and at the same time providing a supporting frame to maintain them in a plane which is always parallel with the rail, a further difficulty is encountered.
Namely, since there can be considerable variations in length of the undulations and the size of hollows and humps which are to be straightened and, particularly when there is a long undulation of considerable amplitude, it can happen that the whole assembly will only rest on two or perhaps only one point, that is at the apex of the undulation. Once everything has been regulated to exert a given pressure on the rail (a pressure normally determined and distributed according to the number of intended rail contacting points) this pressure may occasionally be concentrated entirely at a single, or perhaps only two, rail contacting points.
Thus with a pressure normally distributed, for example, over five grinders, it is highly probable that, at certain times, one of the grinders may have to withstand up to five times its normal working pressure. This can result in the burning of the working surface of the rail and it may severely damage the grinder.